U.S. Pat. No. 3,895,093 issued July 15, 1975 discloses a three-way or equilibrium catalyst specifically designed to handle the exhaust gas stream issuing from an internal combustion engine operating within a very precise and narrow band of air/fuel ratios positioned just slightly on the fuel rich side of stoichiometry. This patent teaches the use of an expensive and sophisticated electronic fuel injection system for metering fuel and air to the engine in the very narrow band of air/fuel ratios required.
In general, equilibrium catalysts are those which attempt to remove most, if not all, of the carbon monoxide, unburned hydrocarbons and oxides of nitrogen present in an exhaust gas stream coming from an internal combustion engine. In the use of this type of catalyst, the air/fuel mixture generally supplied to the engine with which the catalyst is associated is one in which stoichiometric amounts of air and fuel should exist. The catalyst, upon catalytic rearrangement of the undesired components of the exhaust gas stream, eliminates the undesired components by interaction of the components with each other or with the oxygen present in the exhaust gas stream. For example, the catalyst is effective to reduce oxides of nitrogen, thus making available oxygen from this reaction which in turn, can be used in transforming carbon monoxide to carbon dioxide and unburned hydrocarbons to carbon dioxide and water. A stoichiometric air/fuel ratio is generally about 14.7 volumes of air to each volume of fuel although this depends on the exact composition of the fuel being burned.
In using an equilibrium catalyst, there should be just sufficient oxygen present in the air/fuel mixture to burn all of the fuel to the final composition products of carbon dioxide and water. However, in the actual burning of the mixture in the engine, some of the nitrogen present in the air is transformed into oxides of nitrogen, some of the hydrocarbons are not fully burned and some carbon monoxide is formed. Thus, if a stoichiometric air/fuel ratio mixture is burned, even though the products are not the final ones desired, it is merely a matter of rearranging some of the products produced in order to achieve a burning of all of the fuel to its desired final composition products and elimination of all oxides of nitrogen. For example, if the burning in the internal combustion engine is not fully complete and carbon monoxide and unburned hydrocarbons are present, there is sufficient oxygen present in the exhaust gas stream plus the oxygen locked upon the form of oxides of nitrogen to complete the oxidation of the carbon monoxide and unburned hydrocarbons.
U.S. Pat. No. 3,895,093 teaches a specific equilibrium catalyst in which platinum, ruthenium, rhodium and rhenium are present. The patent discloses that this catalyst, with particular ratios of the catalyst elements with respect to each other, is suitable for an equilibrium catalyst. However, the patent discloses that the catalyst must be used in conjunction with an engine running at a precisely controlled air/fuel ratio. This precise control results in the production of an exhaust gas stream having specific concentrations of undesired components having an overall reducing potential.
At this point, we would like to point out that an air/fuel ratio on the rich side of equilibrium means there is an excess of fuel and insufficient oxygen present to burn the fuel, whereas on the lean side of equilibrium there is more oxygen present than is required to burn the fuel present. Thus, the environment in which the catalyst is operating can change from a reducing atmosphere to an oxidizing atmosphere depending on the air/fuel mixture supplied to the engine. We would also like to point out that as a catalyst is subjected to oxidizing and reducing conditions of various potential or strength, the catalyst naturally behaves differently. For example, the reactions which occur under reducing conditions when insufficient oxygen is present over a catalyst are substantially different than those reactions which occur when excess oxygen is available over the catalyst. The reactions also vary as the oxidizing or reducing potential of the exhaust gas stream over the catalyst is increased or decreased.
What the patentees of U.S. Pat. No. 3,895,093 have done is to select a single, operating air/fuel ratio on the fuel rich side of equilibrium and have tailored a precise catalyst to handle the particular composition of exhaust gases produced under the particular set of conditions. In order to use this catalyst as a production catalyst with an automotive engine, it is necessary, as taught by the patentees, to have an elaborate electronic fuel injection system to precisely meter fuel and air to the engine so that the air/fuel ratio varies no more than about 0.1 air/fuel ratio units on either side of the slightly rich air/fuel ratio selected for engine operation. Such an electronic fuel injection system, as is disclosed in the SAE Report 73005 referenced by the patentees, would be complex and costly to manufacture because this system has to have capability to maintain precise air/fuel ratios during such transient vehicle operation modes as acceleration and deceleration.
The patentees of U.S. Pat. No. 3,895,093 do not state in their patent the particular function of the four materials they employ in the disclosed catalyst system. We, however, in view of our long experience in the catalyst field, can state the following with respect to the materials being employed. The patentees teach a very limited operational range which is basically one centered slightly on the rich side of a stoichiometric air/fuel ratio with very minor deviations of no more than 0.1 air/fuel ratio units to either side of the center point. Under these operating conditions we know that the platinum acts as an oxidizing catalyst for oxidizing hydrocarbons and carbon monoxide to end products of water and carbon dioxide. Also, under these conditions the ruthenium principally brings about the reduction of oxides of nitrogen to nitrogen with good selectivity, that is, with little ammonia production. The rhodium is present particularly for the purpose of reducing oxides of nitrogen at near stoichiometric or slightly on the lean side of stoichiometric conditions. The rhenium has good selectivity for the NO to N.sub.2 reaction, but rhenium does not convert substantial amounts of oxides of nitrogen. It is also of interest to note that the patentees have not realized that both ruthenium and rhenium must be protected from volatilization of these materials are exposed to oxidizing ambient. This lack of recognition is because the catalyst operating condition proposed by the patentees is one in which the catalyst is not exposed to oxidizing conditions.
It is a principal object of this invention to provide an equilibrium catalyst which is operative at least momentarily over a range of air/fuel ratios extending approximately at least one air/fuel ratio unit on each side of a stoichiometric air/fuel ratio.
It is still another object of this invention to provide an equilibrium catalyst system which may be used with a present day, reasonably priced, air/fuel ratio control system, such as a carburetor, to provide an overall emission control system which is operable during all operating modes of an internal combustion engine to remove from the exhaust gas stream of the engine substantial quantities of carbon monoxide, unburned hydrocarbons and oxides of nitrogen.